The younger waiter provides a contrast to the older waiter, and the contrast helps develop Hemingway's thesis propounded by the older waiter in his soliloquy. One way this contrast is developed is in their seemingly idle conversation about bed.
(Young waiter): "I'm not lonely. I have a wife waiting in bed for me."
(Older waiter): "Now, without thinking further, he would go home to his room. He would lie in the bed and finally, with daylight, he would go to sleep."
The contrast helps develop Hemingway's thesis that after youth is spent, life is nothing; thus the old ones know despair in the face of this dreadful truth.
(Young waiter and older): "I want to go home to bed."
"What is an hour?"
"More to me than to him. [...] You talk like an old man yourself." [...]
"You have youth, confidence, and a job," the older waiter said."You have everything."
(Older waiter soliloquy): It was a nothing that he knew too well. It was all a nothing and a man was a nothing too. It was only that ... he knew it all was nada y pues nada y naday pues nada.
The old man is a symbol of the nothing/nada theme that the older waiter develops. The old man is a symbol of this because he sits in the dark ("the shadow the leaves of the tree made") even though he sits in light ("against the electric light"): dark is nothing/nada; light is being among the living.
When the younger waiter and the old man who is deaf are taken together, they present an image of Hemingway's thesis